Polymer resins, such as PET, are widely used in the packaging industry. PET is a linear, thermoplastic polyester resin. The myriad advantages of PET include toughness, clarity, good barrier properties, lightweight, design flexibility, chemical resistance and good shelf-life performance. Furthermore, PET is environmentally friendly since it can be recycled. These characteristics of PET make it a popular material in the manufacturing of containers, for example, beverage bottles.
PET beverage bottles are commonly formed using extrusion blow molding (EBM). EBM includes extruding a polymer resin in a softened state through an annular die to form a parason. The parason is placed in a hollow blow mold having a cavity corresponding to the desired shape of the bottle. Air is injected to inflate the parason against the interior walls of the blow mold. Upon contact with the walls, the parason cools instantaneously and permanently assumes the shape of a bottle.
To form beverage bottles, “bottle grade” PET having an inherent viscosity (I.V.) of about 0.72-0.84 dl/g, is typically used. Bottle grade PET has linear polymer chains and can not be used in the production of larger containers such as handleware because of poor melt strength. Low melt strength results in the inability to form a parason. If a parason is not properly formed, the molten parason is drawn down by its own weight, resulting in an hour-glass shape. Parason sagging results in poor material distribution in the walls of the resultant container and an uncontrollable blow molding process.
To counteract the problem of parason sagging, high molecular weight PET having an I.V. of 1.0 dl/g or greater, can be used. The average molecular weight of a resin reflects the average length of polymer chains present therein. In general, melt strength increases with chain length and, thereby, molecular weight. However, higher I.V. requires higher processing temperatures. Higher temperatures cause the resin to thermally degrade resulting in more yellowness in finished parts. The process window narrows making it difficult to run a stable blow molding operation over an extended period. Moreover, longer chain lengths are more susceptible to shear and thermal degradation. These resins also tend to be more expensive than bottle grade PET resin commonly used to produce beverage container packaging, increasing manufacturing cost.
An alternate solution is to use branched PET copolymers. An example is the Eastar Copolyester EB062, manufactured and marketed by Eastman Chemical Company. EB062 is a branched version of a highly modified CHDM (1,4-Cyclohexanedimethanol) copolymer of PET having an I.V. of 0.75 dl/g. Branching effectively increases the melt strength of the resin. Copolymer used in this resin suppresses crystallization producing high clarity while allowing the resin to be processed at lower temperatures. Lower processing temperatures results in higher melt viscosity which in turn serves to improve process stability in extrusion blow molding. Unfortunately, the high levels of copolymers suppress the rate of crystallization to such a level resulting in a virtually amorphous resin affecting recyclability. Amorphous resins, added to the PET recycling stream, causes sticking, agglomeration and bridging issues during the drying process. Amorphous PET when melt processed with bottle grade PET negatively impacts the performance of the recycled resin by suppressing crystallinity and melt temperature, while increasing dye uptake, and reducing physical properties such as hardness, tensile and flexural properties. The severity of the undesirable effects bears a direct relationship to the percentage of amorphous PET content in melt processed recycled PET. As a result, PET copolymers, such as the EB062 are not recyclable at higher concentrations in the PET recycle stream as these resins are more widely used for extrusion blowmolding.
From the foregoing discussion, it is desirable to provide PET blend resins useful for extrusion blow molding and which can be easily recycled.